Nonionic surfactants have broad applications such as cleaning and dispersion stabilization, which frequently are hampered by strong temperature sensitivities. As manifested by clouding and decreased solubility with increasing temperature, the interaction between water and the oligo(oxyethylene) head-groups is becoming less favorable. Different aspects of surfactant self-assembly, like the critical micelle concentration, micelle size and shape, intermicellar interactions and phase separation phenomena are reviewed as well as suggested underlying causes of the temperature dependence. Furthermore, the effect of cosolutes on clouding and the behavior of related systems, non-aqueous solutions and nonionic polymers, are examined.